The present invention relates to a supply circuit.
Supply circuits of this kind serve mainly to supply power to a line circuit, including the subscriber line with the subscriber station. The balanced-to-ground feeding of the supply current through two equal-value resistors is important in order to obtain high crosstalk attenuation between different telephone channels. The feed resistances must be as high as possible for the voice-frequency range, so that the supply circuit will cause only a low bridging loss in the telephone circuit. Therefore, most of the conventional supply circuits, such as the one shown in FIG. 1 of DE-OS No. 25 21 894, contain chokes.
A supply circuit of the kind referred to at the beginning is disclosed in DE-OS No. 30 37 972. It requires no feed chokes. It offers a sufficiently high resistance to push-pull AC signals, so that the bridging loss of the line remains small. However, longitudinal voltages, so-called common-mode AC voltages, may be present on the line running to the variable load. These longitudinal voltages may be audio-frequency voltages, e.g., 16.66-Hz or 50-Hz voltages, or radio-frequency voltages that may damage subsequent circuits, such as the switching network connected to the line. While maintaining the supply current constant independent of such longitudinal voltages, the supply circuit disclosed in DE-OS No. 30 37 972 offers a high resistance to these longitudinal voltages, so that subsequent circuits may be damaged. In addition, the longitudinal voltages may narrow the control range of the supply circuit regulating the supply current. This narrowing of the control range is due to the fact that the supply-circuit transistors, which have a high dynamic resistance, must also suppress the longitudinal voltage. When overdriven by longitudinal voltages, these transistors may, therefore, lose their high resistance to push-pull AC signals (useful signals).